The present invention relates in general to a dynamic balancing device for rotating masses having an imbalanced condition, and more particularly, to a rotating balancing chuck for use in securing a semiconductor wafer thereto and which is constructed and arranged to reduce abnormal vibrations of the wafer which occur due to an unbalanced distribution of the mass of the wafer with respect to the axis of rotation of the chuck.
Semiconductor wafers are spun at high RPM to allow for the performance of a number of process operations, including coating, washing, developing, scrubbing and the like. Wafers of silicon and gallium arsenide crystals are often furnished with one or more flat regions around their perimeters. These flat regions have had two functional purposes. One purpose is to provide for alignment of the crystal structure of the wafer with the rectangular side of the integrated circuits being formed on the wafer itself. Thus, when the wafers are diamond scribed, they will break evenly and the cracking process will follow along the crystal grain. On the other hand, when such wafers are sawed to separate the IC chips, crystal grain alignment is not as critical. Wafer flat regions, however, are still in wide use today as a means of rough alignment orientation between subsequent patterns applied by alignment and exposure equipment.
As wafer sizes have increased over the years to 150 mm, and even 200 mm along with increases in thickness, the effect of weight imbalance due to these wafer flat regions on spinning equipment has become more pronounced. When such wafers are spun at high RPM, vibrations are created which have a number of adverse effects. For example, vibrations are transmitted to other parts of the equipment, such as wafer cassettes, causing position shifting of the wafer and causing photoresist flaking. In addition, these vibrations cause undue wear on all bearings and slides of the spinning equipment due to chatter, thereby necessitating frequent maintenance and repair. As a consequent of these vibrations, unexpected resonances get built into the spinning equipment, as the equipment is modified for other reasons. Further, sounds emitted during the spinning operation are distracting and annoying to operating personnel, thus detracting from the overall image of quality of the equipment.
There are known a number of automatic dynamic balancing techniques for dampening vibrations in rotating masses, such as rotors, shafts and the like. Typical of such techniques is the construction and arrangement of an automatic balancer of the type having a plurality of spherical counterweights or ball weights, in an annular race rotating about an axis, for example, as disclosed in any one of U.S. Pat. Nos. 4,433,592, 4,075,909, 4,060,009, 3,970,260, 3,799,619, 3,733,923, and 3,410,154. As known from these automatic balancers, the spherical counterweights are arranged circumferentially and movable in the annular race, and as the object to be balanced rotates about its axis, move to the side opposite to that on which the unbalanced load of the object is located to absorb the unbalanced condition. However, these automatic balancers are generally relatively complex in construction and are adapted only to grossly compensate for large mass imbalances in rotating objects. When semiconductor wafers are employed, the mass imbalance is often small and requires precision load compensation which cannot be achieved by these known automatic balancers. There is also known from U.S. Pat. Nos. 2,861,471 and 209,475 a balancing device which employs a securable counter mass, such as a plurality of bolts or metal segments, securely arrangeable within a race for compensation of mass imbalance due to a rotating object. However, these balancing devices also possess the aforementioned disadvantages.